This invention relates generally to holograms in photorefractive materials, and more particularly has reference to a new and improved method for promoting enhanced nondestructive reconstruction.
Holograms have been widely used for a variety of optical processing and data storage applications. For example, it is well known that information can be stored in a hologram by optically encoding the object recording beam. Increased data storage capacity is achieved by selectively shifting the directions of the recording beams in a manner which produces a plurality of spatially multiplexed holograms within the same recording element.
Isotropic materials, such as dichromated gelatin, have long been used as recording media for various types of amplitude and phase holograms. A thin film of the material is applied to a substrate, and the hologram is recorded by an opto-chemical photographic process. The post-exposure chemical development slows the recording process, and the thinness of the recording film reduces the data storage capacity. The chemical development also makes it difficult to use this type of hologram for computer data storage applications.
Ferroelectric photorefractive materials, such as strontium barium niobate (SBN) and lithium niobate (LiNbO3), have been investigated as alternative holographic recording media. A relatively thick crystalline form of the material is used, and the hologram is recorded by an electro-optical process which can occur in the millisecond or microsecond regime, or even faster using pulsed laser sources.
The usual model for photorefractive recording in non-linear optical materials explains the process as a photoelectric excitation of donor electrons into the conduction band, followed by charge migration and subsequent trapping of the charge carriers, resulting in a space charge distribution within the material which is related to the light intensity distribution of the interference pattern set up by the recording beams. The magnitude of the space charge field initially increases with recording energy, and then approaches asymptotically a saturation value. The excited donor electrons are temporarily trapped at trap sites producing localized changes in the index of refraction of the material. These changes are a function of the induced space charge field, the external voltage gradient applied across the crystal, and the electro-optical parameters of the material, particularly the dielectric tensor and the electro-optic tensor. The physical mechanism is defined by the following equation, which indicates the change in the optical susceptibility of the crystal (.epsilon.) caused by the space charge field (E): EQU .DELTA..epsilon.=-.epsilon..sub..omega. .multidot.(r.multidot.E).multidot..epsilon..sub..omega.
(where .epsilon..sub..omega. denotes the dielectric tensor and r is the electro-optic tensor).
The resulting variations in the index of refraction of the material define a recorded hologram. When the material is then re-illuminated with a reconstruction beam, these variations in the index of refraction produce phase modulations in the light, resulting in a reconstruction beam which reconstructs in optical form the information stored in the hologram.
Different reconstruction polarizations (relative to the crystal axis of the recording material) produce different reconstruction efficiencies, .eta., as determined by the optical susceptibility tensor. Jones Calculus can be used to calculate these different efficiencies, as follows: ##EQU1## where P.sub.IN and P.sub.OUT are the reconstruction input and the reconstructed output Jones Vectors and .DELTA..epsilon. is given by the equation set out in the preceeding paragraph.
A difficulty with holograms in photorefractive media is the problem of destructive reconstruction. Where the recording cycle is substantially symmetrical with the reconstruction cycle, the re-illumination reference beam (i.e., the reconstruction beam) which is used to retrieve the recorded information excites the donor electrons and disturbs the equilibrium of the space charge field in a manner which produces a gradual exponential erasure of the recording. This reduces the number of reconstructions that can be made before the signal-to-noise ratio becomes too low. Moreover, because recordings are similar to reconstructions in terms of electron excitation, each recording can degrade earlier recordings in the same region of the recording medium. This restricts the ability to use the three-dimensional capacity of a photorefractive crystal for recording spatially multiplexed holograms. An ideal recording/reconstruction cycle would be asymmetrical, that is, it would take more energy to erase a recording than is used to make the recording in the first place.
A number of investigations have been carried out regarding the photorefractive phenomenon and architectures for repeated data recording and reconstruction of optically encoded information. Recycling is the usual means suggested for permanent data storage. However, several techniques have been suggested for fixing the hologram to achieve repeated reconstruction without recycling. Heat fixing and electrical fixing are two examples.
Heat fixing involves heating the crystal above the Curie temperature during the recording phase and subsequently cooling below the critical temperature so that the electric field patterns of the hologram induce corresponding polarization domains which are stable at room temperature. The recording is erased by again heating above the Curie temperature.
Electrical fixing involves a procedure by which the hologram is first poled to align all polarization domains with a field well above the coercive field. A hologram is recorded and is then fixed by applying an electric field antiparallel to the original poling field. Polarization reversal occurs at those locations where the sum of the space charge field and the applied field is above the coercive field, resulting in a replication of the original trapped charge pattern. This pattern masks the holographic space charge field, and the efficiency of the reconstruction is initially low. Upon re-illumination with the reference beam, the charges redistribute themselves to reveal the domain pattern. The ultimate efficiency is high (often higher than the original efficiency before the switching field was applied), caused by overcancellation of the space charge field by local polarization switching. The recording is erased by applying another poling field.
The techniques suggested by these prior investigations have limited utility because they involve substantial extraneous processing steps and/or equipment. A need exists for a more convenient and effective method for promoting an asymmetrical recording/reconstruction cycle in photorefractive media. Ideally, such a method also would provide an enhanced reconstruction in which the efficiency actually increased above its starting value. The present invention fulfills these needs.